A two-stage compression refrigeration system conducts a compression process in two stages, i.e., increasing intermediate pressure between condensing pressure and evaporating pressure; and low-voltage refrigerant vapor from an evaporator is firstly compressed from evaporating pressure at a low-pressure stage of the compressor into appropriate intermediate pressure, then enters a high-pressure stage after being intercooled, and is compressed again from the intermediate pressure into the condensing pressure, thereby forming two-stage compression. A cascade refrigeration system consists of two refrigeration systems, respectively known as a high-temperature portion and a low-temperature portion. The high-temperature portion uses an intermediate pressure refrigerant and the low-temperature portion uses a low-temperature and high-pressure refrigerant. An overlapped device of the high-temperature portion and the low-temperature portion is a condensation evaporator which is an evaporator of the high-temperature portion as well as a condenser of the low-temperature portion. In the condensation evaporator, an intermediate temperate refrigerant of the high-temperature portion performs vaporization and heat absorption for condensation of the refrigerant of the low-temperature portion.
In refrigeration engineering, when evaporating temperature reaches a temperature below −25° C., only a small refrigeration device still adopts a single-stage compression refrigeration system in order to simplify the system, but the minimum temperature can only reach −40° C. In a large system for, e.g., freezing processing of food, when the evaporating temperature of −30° C. to −60° C. is prepared, a two-stage compression refrigeration system is generally used and when the evaporating temperature of −60° C. to −80° C. is required to be prepared, the two-stage compression refrigeration system often cannot satisfy the requirement due to the limitation of such factors as refrigerant solidifying point, system pressure ratio, evaporating pressure, operational economics, etc. At this moment, a cascade refrigeration system is required to be adopted. That is: the evaporating temperature of the two-stage compression refrigeration system is generally regulated as −30° C. to −60° C., and the evaporating temperature of the cascade refrigeration system is generally regulated as −50° C. to −80° C.
To extend a section of refrigeration temperature of the cascade refrigeration system, a patent documentation with the publication No. of CN202973641U discloses a −80° C. series-parallel automatic switching cascade refrigeration system which comprises a high-temperature level refrigeration system and a low-temperature level refrigeration system. An outlet of a high-temperature level compressor is communicated with a liquid storage tank through a high-temperature condenser; an outlet of the liquid storage tank is divided into two paths through a drying filter; an outlet of the low-temperature level compressor is divided into two paths; one path of an outlet of an expansion vessel is communicated with an inlet of the low-temperature level compressor; the other path is communicated with a low-temperature evaporator through a tubular exchanger; and an outlet of the low-temperature evaporator is communicated with an inlet of the low-temperature level compressor through an oil separator. The system during operation respectively realizes temperature control of high-temperature level refrigeration (room temperature to −40° C.) and low-temperature level refrigeration (−40° C. to −80° C.) by switching solenoid valves, so as to realize temperature control from room temperature to −80° C., thereby obtaining large scope of refrigeration section, increasing the operating efficiency of the compressor and reducing operating cost. However, because the high-temperature level of the above refrigeration system adopts the single-stage compression refrigeration system, as mentioned previously, in the refrigeration engineering, when the evaporating temperature is below −25° C., corresponding evaporating pressure is also low and the pressure ratio pk/po is too large, often leading to greater deviation of an actual compression process of the compressor from an isentropic degree, thereby increasing actual power consumption of the compressor and decreasing the efficiency; overlarge pressure ratio may also result in an increase in exhaust gas temperature of the compressor, while overhigh exhaust gas temperature will result in thinning and even carbonization of lubricating oil. Therefore, the single-stage compression refrigeration system is not adopted.
At present, a conventional defrosting mode of an air cooler is to adopt traditional electrical heating for defrosting. Defrosting time is controlled by a defrosting controller, and an electrical heating wire generates radiant heat for melting a frost layer. Such a method has the disadvantages: a defrosting system consumes large power; moreover, an electrical heating system has many elements; defrosting is inadequate so that the safety of a product is reduced. In practical situations, large fluctuation of storehouse temperature is often caused, and the storage quality of the food is affected.